1. Field of the Invention
The present invention relates to a process for producing a low-attenuation optical fiber.
2. Description of the Related Art
One of the most important characteristics of a telecommunication optical fiber is the signal attenuation in particular wavelength regions currently used for long-distance transmission. In fact, the lower is the signal attenuation, the longer the distance the signal can travel before being amplified.
It is well known that fiber attenuation is negatively affected by the presence of impurities, which can be incorporated within the fiber during the process of manufacturing thereof. Particularly troublesome is the attenuation caused by contamination by hydroxyl radicals (OH) or water. The attenuation increase due to the presence of OH or water in the glass can be as high as about 0.5 to 1.0 dB/km, with a peak in the wavelength range from 1330 nm to 1470 nm (commonly identified as “1380 nm window”), making this range unsuitable for long-distance transmission. The most suitable wavelength range for long-distance transmission is still that around 1550 nm, which guarantees very low losses.
The advent of wavelength division multiplexing (WDM) technology, which enables telecommunication systems to operate over broad wavelength ranges, makes it likely to exploit the 1380 nm window. Removing, or at least reducing, the water peak from optical fibers is very important to obtain low-loss optical telecommunication systems operating on a wide wavelength band (such as from about 1300 nm to about 1700 nm).
WO 00/64825, in the attempt to solve the above problem, proposes a method of fabricating a cylindrical glass body for use in manufacturing optical waveguide fiber, wherein the incorporation of water is at least reduced.
WO 00/64825 describes the following process to produce an optical fiber. A porous body is made by depositing reaction products on a cylindrical substrate in a conventional Outside Vapor Deposition (OVD) process. A centreline hole (or “central hole”) extending axially through the porous body is formed by removing the substrate. The hollow body so obtained is consolidated in a chlorine-containing atmosphere to chemically dry the blank, thus forming a consolidated glass preform having a centerline hole extending axially therethrough. The core glass preform is then typically positioned within a redraw furnace and heated to a temperature sufficient to facilitate redrawing or stretching of the core preform into a smaller diameter cylindrical glass body or core cane. During the redraw operation, the centerline hole of the core blank is closed by, for example, applying vacuum along the centerline hole. The reduction in pressure within the centerline hole ensures complete closure of the centerline hole such that the core cane has a solid centerline region extending axially therethrough. After the redraw step, the resulting core cane is typically overclad with a layer of cladding soot, e.g. deposited via an OVD process. The resulting soot overclad core cane is chemically dried and consolidated to form an optical fiber preform, which can thereafter be drawn into an optical waveguide fiber.
Despite the chemical drying and consolidation steps, such optical fibers have been found to exhibit a relatively high level of attenuation measured at approximately 1380 nm. The water peak is largely a result of water being trapped in the glass during the fiber manufacture process, a large portion of this water being trapped within the centerline region of the core cane prior to or during closure of the centerline hole. In fact, although the blanks are chemically dried and sintered during consolidation, it has been found that the region of glass surrounding and defining the centerline hole is being rewet after drying, mainly due to exposure to ambient atmosphere, which unavoidably contains water. This rewetting can occur for example when the preform is removed from the consolidation furnace and moved to the redraw furnace for further processing steps. Moreover, the greater the exposure time, the greater the amount of water absorbed by the glass.
To reduce the amount of water trapped within the centerline region of the core cane, WO 00/64825 proposes either to prevent water exposure of the centerline hole of the dried and consolidated preform by closing the centerline hole during consolidation, or to chemically remove the water after rewetting has occurred, preferably at redraw, by treating the centreline hole with a chemical drying agent, a chemical etching agent or deuterium.
In order to prevent rewetting of the glass bounding the centerline hole, it is proposed either to close the centerline hole or to seal the centerline hole during consolidation.
As concerns the solution of closing the centerline hole, the following method is described. Prior to consolidation of the soot preform, a glass plug is fitted to the end of the centerline hole opposite the end of the soot preform provided with a handle. Following chlorine drying, the porous body (held vertically via the handle) is down driven into the hot zone of the consolidation furnace, preferably in an inert gas atmosphere, such as helium. The elevated temperature generated in the hot zone, preferably about 1500° C., sinters porous body as it enters the hot zone. The inwardly directed sintering forces reduce the diameter of porous body thereby closing porous body onto plug to effectively seal one end of centerline hole. The porous body is further down driven to sinter the remainder of porous body thereby forming a sintered glass preform having a centerline hole sealed at its plugged ends.
Following the consolidation step, the sintered glass preform is preferably withdrawn from the hot zone, and the centerline hole is exposed to a vacuum of at least 10 Torr, more preferably 100 mTorr, through an inner handle, which communicates with centerline hole through the handle. The sintered glass preform is again down driven into the hot zone of consolidation furnace while centerline hole is under vacuum. As the sintered glass preform enters the hot zone, it softens sufficiently so that the vacuum force acting on the glass bounding the centerline hole draws the glass inward, thereby closing centerline hole as the sintered glass preform continues to move through the hot zone.
The resulting solid sintered glass preform can then be removed from consolidation furnace and stored for further processing at a later time, or moved to a redraw furnace where it can be drawn into a reduced diameter cane. In either event, since centerline hole is closed (i.e., the sintered glass preform has a solid centerline region), the centerline region will not be exposed to ambient atmosphere and thus will not be rewet upon removal from consolidation furnace.